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Hypersensitivity Pneumonitis (HP)

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Hypersensitivity pneumonitis (HP)

Hypersensitivity pneumonitis (HP) is an immune-mediated inflammatory lung disease caused by repeated inhalation of an environmental antigen in a susceptible person.

Also called

Extrinsic allergic alveolitis (EAA) — older term

ICD-10

J67.9

Specialty

Pulmonology

Onset

Chronic

Reviewed

July 2026

On This Page

Overview

Hypersensitivity pneumonitis (HP)  occurs when inhaled antigens trigger an abnormal immune response in the:

  • Alveoli
  • Small airways
  • Lung interstitium

Common sources include:

  • Birds
  • Mold
  • Fungi
  • Agricultural dusts
  • Contaminated water or humidifiers

Repeated exposure may cause:

Inflammation → Lung injury → Fibrosis

Etiology & Risk Factors

Common Causes of Hypersensitivity pneumonitis (HP)

Bird Exposure

  • Bird feathers
  • Bird droppings
  • Bird proteins

Often called bird fancier’s lung.

Mold & Fungi

  • Moldy hay
  • Damp buildings
  • Compost
  • Contaminated ventilation systems

Agricultural Exposure

  • Hay
  • Grain
  • Farming dusts

Often associated with farmer’s lung.

Other Exposures

  • Humidifiers
  • Hot tubs
  • Contaminated water systems
  • Certain occupational chemicals

Risk Factors

  • Repeated antigen exposure
  • High-intensity exposure
  • Farming
  • Bird keeping
  • Working with mold or organic dust
  • Certain occupational environments
  • Genetic susceptibility

Pathophysiology

Inhalation of environmental antigen
⬇
Antigen reaches the small airways and alveoli
⬇
Abnormal immune response
⬇
Lymphocytic inflammation
⬇
Small airway + Interstitial lung injury
⬇
Granuloma formation in some patients
⬇
Repeated or persistent exposure
⬇
Chronic inflammation
⬇
Pulmonary fibrosis

Key Concept

Early inflammatory disease may improve after antigen avoidance, while established fibrosis may be irreversible

Clinical Presentation

  1. Acute Features of Hypersensitivity pneumonitis (HP)

    Symptoms may develop within hours after heavy exposure:

    • Fever
    • Chills
    • Dry cough
    • Dyspnea
    • Fatigue
    • Malaise

    Symptoms may improve after leaving the exposure.

    Chronic Features of Hypersensitivity pneumonitis (HP)

    • Progressive exertional dyspnea
    • Persistent cough
    • Fatigue
    • Reduced exercise tolerance
    • Weight loss

    Signs

    • Tachypnea
    • Fine inspiratory crackles
    • Reduced oxygen saturation

    Advanced Disease

    • Digital clubbing
    • Cyanosis
    • Signs of pulmonary hypertension
    • Signs of respiratory failure

History Taking

      • Shortness of breath?
      • Dry cough?
      • Fever or chills after exposure?
      • When did symptoms start?
      • Do symptoms improve away from home or work?
      • Bird exposure?
      • Feather pillows or bedding?
      • Mold or dampness at home?
      • Farming or hay exposure?
      • Hot tub use?
      • Humidifier use?
      • Occupational exposure?

Physical Examination

  • Look for:

    • Tachypnea
    • Respiratory distress
    • Reduced oxygen saturation
    • Cyanosis

    Chest Examination

    May show:

    • Fine inspiratory crackles
    • Inspiratory squeaks

    Chronic or Advanced Disease

    Look for:

    • Digital clubbing
    • Signs of pulmonary hypertension
    • Signs of cor pulmonale

Investigations of Hypersensitivity pneumonitis (HP)

  • High-Resolution CT (HRCT) Chest — Key Imaging Test

    May show:

    • Ground-glass opacities
    • Centrilobular nodules
    • Mosaic attenuation
    • Air trapping
    • Fibrosis in chronic disease

    Important Imaging Feature

    The combination of:

    • Ground-glass changes
    • Mosaic attenuation
    • Air trapping

    supports the diagnosis of HP.

    Pulmonary Function Tests

    May show:

    • Restrictive pattern
    • Obstructive pattern
    • Mixed pattern
    • Reduced DLCO

    Blood Tests

    May include:

    • Serum antigen-specific IgG antibodies
    • Inflammatory markers

    Important Note

    Positive antigen-specific IgG indicates exposure but does not alone confirm the diagnosis.

    Bronchoalveolar Lavage (BAL)

    May show:

    • Increased lymphocytes

    Lung Biopsy

    Consider when:

    • Diagnosis remains uncertain
    • Clinical and imaging findings are inconclusive

Diagnosis of Hypersensitivity pneumonitis (HP)

Hypersensitivity Pneumonitis · Diagnostic Confidence Levels & Stepwise Workup — ATS 2020

Diagnostic Confidence Levels — ATS 2020
High confidence / Definite
HP confirmed
Typical HRCT + identified antigen + BAL lymphocytosis ≥20% OR typical histopathology on biopsy. No need for further workup. Start management.
Moderate confidence
Probable HP
Compatible HRCT + antigen exposure + BAL lymphocytosis, OR typical HRCT without identified antigen. MDT review recommended. Biopsy to consider.
Low confidence
Possible HP
Indeterminate HRCT ± unclear antigen exposure. Biopsy usually required for diagnosis. Exclude alternative diagnoses (IPF, CTD-ILD, drug-induced).
Stepwise Diagnostic Workup
History & antigen identification
Detailed exposure history — occupational, home, hobbies; ask specifically about birds, feather duvets/pillows, hot tubs, mouldy environments, farming
Temporal relationship — symptoms worse at work or home? Improve on weekends or vacation? (classic HP pattern)
Serum-specific IgG (precipitins) — against avian, fungal, bacterial antigens; positive in ~90% of acute/subacute HP; less sensitive in chronic HP
Precipitin positivity = sensitisation (not disease); ~10% of exposed individuals without HP may be seropositive
Home / workplace visit or environment assessment if antigen unclear
Controlled antigen inhalation challenge — specialist centres only; rarely needed; can precipitate severe reaction
HRCT & BAL
HRCT — inspiratory + expiratory views — expiratory views essential to detect air trapping (mosaic attenuation — pathognomonic feature of HP)
Classic non-fibrotic HP HRCT triad:
1. Centrilobular ground-glass nodules (poorly-defined, <5 mm)
2. Bilateral ground-glass opacification (upper/mid-lung)
3. Mosaic attenuation + air trapping on expiratory views
Fibrotic HP — adds upper lobe fibrosis, traction bronchiectasis; honeycombing may occur
BAL — lymphocytosis ≥20% strongly supportive; >50% highly suggestive
CD4:CD8 ratio <1 (inverted — opposite to sarcoidosis); BAL mast cells may be elevated
Spirometry — restrictive or mixed pattern; DLCO reduced; air trapping (RV/TLC ratio ↑)
Biopsy & MDT
Biopsy indicated if: diagnosis uncertain after HRCT + BAL; fibrotic HP vs IPF distinction critical for management
Transbronchial cryobiopsy (TBLC) — preferred; diagnostic yield ~70–80% for HP
VATS surgical lung biopsy — gold standard if cryobiopsy non-diagnostic
Histopathological triad of HP:
1. Cellular bronchiolocentric interstitial pneumonia
2. Non-necrotising poorly-formed granulomas near bronchioles
3. Giant cells (foreign body or Langhans type)
MDT review — pulmonologist + radiologist + pathologist — essential for unclassifiable or complex cases; improves diagnostic accuracy
Home / Home / Clinical Cases / Respiratory System Clinical Cases / Hypersensitivity Pneumonitis (HP)

Related Topics

Management Hypersensitivity pneumonitis (HP)

  • Main Goals of Management of Hypersensitivity pneumonitis (HP)

    • Identify the responsible antigen
    • Stop further exposure
    • Reduce lung inflammation
    • Prevent pulmonary fibrosis

    Antigen Avoidance — Most Important Treatment

    • Remove the responsible exposure
    • Improve ventilation
    • Remove mold contamination
    • Avoid responsible birds or animals
    • Use appropriate workplace protection

    Important Note

    Complete antigen avoidance is the cornerstone of treatment.

    Corticosteroids

    May be used in patients with:

    • Significant symptoms
    • Severe inflammatory disease
    • Progressive disease

    They may improve symptoms and speed recovery but do not replace antigen avoidance.

    Fibrotic Disease

    Management may include:

    • Treatment of progressive pulmonary fibrosis
    • Antifibrotic therapy in selected patients

Complications

  • Chronic pulmonary fibrosis
  • Progressive loss of lung function
  • Chronic hypoxemia
  • Respiratory failure
  • Pulmonary hypertension
  • Cor pulmonale
  • Acute exacerbation
  • Death

Prognosis of Hypersensitivity pneumonitis (HP)

  • Prognosis depends mainly on:
    • Early identification of the antigen
    • Successful antigen avoidance
    • Presence or absence of fibrosis
    • Severity of lung impairment
  • Nonfibrotic HP may improve significantly after exposure stops.
  • Fibrotic HP may continue to progress despite antigen avoidance.
  • Established pulmonary fibrosis is associated with a worse prognosis.
  • Early diagnosis and complete exposure avoidance improve outcomes.

Key Points / Clinical Pearls of Hypersensitivity pneumonitis (HP)

  • Hypersensitivity pneumonitis (HP) is an immune-mediated lung disease caused by inhaled environmental antigens.
  • Birds, mold, and farming exposures are common causes.
  • A detailed exposure history is essential.
  • Symptoms may worsen after exposure and improve when away from it.
  • HRCT is the key imaging test.
  • BAL often shows lymphocytosis.
  • Positive antigen-specific IgG indicates exposure, not necessarily disease.